In a conventional method, the fuel is conveyed from a fuel tank into a pressure region by an electrical fuel pump. A fuel injector is connected to this region. This injector is in turn positioned in an intake manifold of the internal combustion engine. In this manner, the fuel may reach the intake manifold via the fuel injector and from there reach the combustion chambers of the internal combustion engine. A further method of the type initially cited is known from internal combustion engines which operate using gasoline direct injection. In these internal combustion engines, the fuel is conveyed by an electrical fuel pump, which is also referred to as a “presupply pump,” from the fuel tank into a pressure region, and from there reaches a high-pressure fuel pump (“main supply pump”), which is generally mechanically driven. This pump conveys the fuel further into a common fuel line (“rail”). Multiple injectors are connected to this rail, and the fuel is stored at high pressure therein. The injectors each inject the fuel directly into the corresponding combustion chambers of the internal combustion engine.
If the electrical fuel pump and the pressure region positioned downstream from it are configured as a “constant-pressure system,” the pressure region is connected via a mechanical pressure regulator to the fuel tank. In normal operation, the electrically driven fuel pump conveys the fuel continuously and at the maximum output rate. In the known internal combustion engines and/or the known methods, any quantity of fuel which is not sprayed into the intake manifold by the fuel injector in systems having intake manifold injection, and which is not conveyed further by the high-pressure pump in systems having gasoline direct injection, flows back into the fuel tank via the mechanical pressure regulator.
Since the electrically driven fuel pump runs continuously at the maximum output rate, it is ensured that the pressure in the pressure region always remains at the desired level, even if the maximum possible quantity of fuel is demanded by the fuel injector and/or the injectors.
Demand-controlled fuel systems are also known. These are also constant-pressure systems, in which the pressure in the pressure region is set to a constant value through the activation of a mechanical pressure regulator. The fuel pump is therefore no longer activated fully, i.e., continuously at maximum output, but rather only according to the demand of the internal combustion engine. The excess quantity of fuel flows back into the tank via a mechanical pressure regulator. The adjustment of the conveyance output to the instantaneous operating point of the internal combustion engine causes a savings in fuel, since the drive output of the electrically driven fuel pump may be reduced in many operating ranges of the internal combustion engine.
During startup of the internal combustion engine, sufficient pressure must be provided in the pressure region of the fuel system so that the fuel reaches the combustion chambers of the internal combustion engine in the desired manner. Typically, it is assumed that the pressure of the fuel in the pressure region falls to ambient pressure after the internal combustion engine is shut off. In order to be able to achieve a desired pressure for starting the internal combustion engine, at least the quantity of fuel necessary for compressing the fuel to the desired pressure must therefore be conveyed. The expansion of the fuel system during the pressure buildup must also be taken into consideration. In some known methods, the operating time of the fuel pump, which is driven at constant output, during the prerun is a function of the period of time which has passed since the internal combustion engine was shut off.
Using the shutoff time of the engine, a fuel system pressure, the number of pump preruns which have already occurred, etc., for example, as criteria for requiring a fuel pump prerun is described in German Published Patent Application No. 199 61 298.
German Published Patent Application No. 100 14 550 describes the possibility of controlling the fuel pressure during the prerun on the basis of a pressure sensor by changing the speed of the fuel pump.
In this method, the conveyance output of the electrically driven fuel pump during the prerun is tailored to the particular demand. This demand is defined by the signal provided by the pressure sensor. If the pressure sensor signals that the pressure in the pressure region is lower than desired, the electrical fuel pump is activated accordingly. In contrast, if the pressure sensor signals that the pressure in the pressure region already corresponds to the desired pressure, the electrical fuel pump remains switched off.